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Peavey Renown

Started by Hawk, June 26, 2015, 09:12:28 AM

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DrGonz78

#60
Try to just measure each side of the bridge rectifier red probe on (~) AC in to bridge and black probe to ground. Give us the voltages that way to be clearer.

Also, an infinite reading would indicate an open condition, as in infinite resistance. A shorted reading would simply be low resistance, such as the same reading as measuring a piece of wire. Use your ohms setting on your meter and if it is a low resistance (hence 1ohm or less) then it is shorted. Although measuring a capacitor we should see a very high resistance.

Edit: Note that we still need to know if that C69 is shorted. Measuring the voltage at the bridge, the way you did, is also a correct way to go about this too. I am concerned about that C69 still. If the secondary leads are connected you will not get an accurate reading and you will have to lift one leg of C69 to do an ohms test.
"A person who never made a mistake never tried anything new." -Albert Einstein

Hawk

Thanks! Up early for work. Will post tomorrow :tu:

Enzo

Don't focus on that 1.4v, it tells us nothing useful.

You have SOMETHING loading down the supply, that is why the bulb is bright.  Have you removed the shorted output or outputs? Oh, you said you did. If they are still in there, then that explains it, at least in part.  The schematic shows an output matching transformer.  If you have this in your amp, unplug it from the board.  And remember, we are using NO speaker or load.  If the output transformer is unhooked, NOW does the bulb act the same?

About the power transformer, it is supposed to provide 60v red to red, but that is center tapped, so it is important that the center tap works.  So with the red wire connector pulled off, measure AC volts from the center pin to either end, and you ought to get roughly 30v.  Yes, a little less with the bulb in place.   I suspect it is OK.


The rectifiers are now new?  so that ought to be good.  Assuming they are in the right way.  If neither rail is shorted to ground by C66, C67, then the power amp has the problem.  A shorted filter cap there is pretty rare. 

You checked the outputs, found and replaced a shorted one.  But did you also check all the 0.33 ohm 5 watt resistors associated with them?  They will either be open or they will be OK.  Also the three 47 ohm resistors asociated with the 5331 and 5332, are they open?  Those two transistors, Q5, Q9 are your drivers.  They are often damaged when outputs blow.  Check them, I usually replace them when I have to replace outputs.

A little left of those are CR26, CR27 diodes.  Make sure neither is open.  CR26 is a dual one - two diodes in one part.  So on your meter, it will look more like 1v on diode test instead of 0.5v.  CR27 is a regular diode.

My suspicion is that if you have the output matching transformer, it may be the cause of the bulb lighting, so do unplug it.  I don't think it is what is wrong, but it will cause the bulb to light if the amp is making DC.

For the power amp to draw current like that, we either have both plus and minus sides turning on at once, or we have shorted something in the output stage.

Hawk

#63
QuoteAnd remember, we are using NO speaker or load.  If the output transformer is unhooked, NOW does the bulb act the same?
Yes, acts the same.

QuoteSo with the red wire connector pulled off, measure AC volts from the center pin to either end, and you ought to get roughly 30v.  Yes, a little less with the bulb in place.   I suspect it is OK.
27VAC

QuoteThe rectifiers are now new?  so that ought to be good.  Assuming they are in the right way.  If neither rail is shorted to ground by C66, C67, then the power amp has the problem.  A shorted filter cap there is pretty rare.
Yes, rectifiers new


Dr. Gonz:Edit:
QuoteNote that we still need to know if that C69 is shorted. Measuring the voltage at the bridge, the way you did, is also a correct way to go about this too. I am concerned about that C69 still. If the secondary leads are connected you will not get an accurate reading and you will have to lift one leg of C69 to do an ohms test.
Removed one leg and got an infinite reading. Checked other good caps out of circuit and got an infinite reading so I guess is infinite is normal for a good cap.

QuoteBut did you also check all the 0.33 ohm 5 watt resistors associated with them?  They will either be open or they will be OK
They test OK.

QuoteThose two transistors, Q5, Q9 are your drivers.  They are often damaged when outputs blow.  Check them, I usually replace them when I have to replace outputs
I replaced them as I buggered them up when I removed for testing. Brand new. Only used with the light bulb limiter.

QuoteAlso the three 47 ohm resistors asociated with the 5331 and 5332, are they open?
They all show 47 ohms.

QuoteA little left of those are CR26, CR27 diodes.  Make sure neither is open.  CR26 is a dual one - two diodes in one part.  So on your meter, it will look more like 1v on diode test instead of 0.5v.  CR27 is a regular diode.
CR26 .015 V on diode test, both directions. CR27 .515 one direction, 0L the other.


Thanks again.




DrGonz78

Looks like CR26 is shorted out. Got to replace that and then just keep checking as many components as possible. I am glad Enzo knew more about that output transformer. I guess I was surprised to see an output transformer on this amp. If nothing else so far it seems you are getting better at testing for bad parts.
"A person who never made a mistake never tried anything new." -Albert Einstein

Enzo

The power amp works without the transformer, but it allows full power at both impedances.  However, when the amp makes DC, that trasnsformer then acts like a dead short.  So we disconnect it.

Actually I was more worried that CR26 was open.  All shorting it does is make the amp run cooler and add some crossover distortion.

Hawk

Thanks again guys.
Quote
The power amp works without the transformer, but it allows full power at both impedances.
So, by two do you mean with the transformer connected (one impedance) and not connected (another impedance, or lacking impedance because it's not connected)? You say the power transformer works without the transformer, but you mean not properly, correct, otherwise we wouldn't have a problem?

Tonight, I'll comb through every component on that power amp. Easy to want to give up at this point but I'll keep at it...................


phatt

Quote from: Hawk on August 02, 2015, 07:29:31 AM
Thanks again guys.
Quote
The power amp works without the transformer, but it allows full power at both impedances.
So, by two do you mean with the transformer connected (one impedance) and not connected (another impedance, or lacking impedance because it's not connected)? You say the power transformer works without the transformer, but you mean not properly, correct, otherwise we wouldn't have a problem?

Tonight, I'll comb through every component on that power amp. Easy to want to give up at this point but I'll keep at it...................

It's an impedance matching Auto transformer, yes the Amp would likely work fine without it but those older peavey's had many little extras.
Phil.

phatt

Hopefully without adding confusion,,,

OK if the amp runs colder with CR26 shorted then rail voltage *Should* go Up not down to 2 or 3 volts.???

With such low rails there has to be a major issue somewhere else. I may have missed it but, Have the main filter caps been checked? My old Laney combo came to me dead, instantly blowing fuses.
The main filter caps where the cause. Laney was likely similar age.

Hey Enzo What if?

As the open circuit AC voltage reads correct when not connected to board but returns very low DCV when reconnected to Board then;
1/ Main Filter Caps are dead or dying.
2/ A short on both rails.
(would I be right in thinking for voltages to drop that low surely the transformer would be humming/buzzing very loudly even with the limiting lamp)

Or 3/
Many years back (before I had much of a clue) while testing many different poweramp designs I did run into a rather odd problem which to this day I still don't know the cause but my supply transformer gave similar fault conditions as presented here.

Replaced all 4 diodes (Bridge) and both Caps,, same result. :grr :grr

With no circuit connected the DCV read 37 Volt rails but the moment you connected a load the DCV dropped like brick, close to zero DCV.
This was over 20 years back so I can't recall exact details but I don't think the transformer even hummed.

Just Maybe a bad winding would cause that kind of condition. xP

Replaced the transformer and all was normal.
It really ticked me off because that transformer was a big iron with large cross section and bell ends. (from a big Sansui Amp)

Lets hope it's not the transformer. :tu:
Phil.

Hawk

#69
QuoteWith no circuit connected the DCV read 37 Volt rails but the moment you connected a load the DCV dropped like brick, close to zero DCV.
This was over 20 years back so I can't recall exact details but I don't think the transformer even hummed.
Thanks for that Phatt. I'll do a transformer test when I get home from work.I believe I used this method before on this transformer but that was a while back. I'll re-test for sure. http://www.premierguitar.com/articles/the-super-secret-transformer-tester-1

QuoteWith such low rails there has to be a major issue somewhere else. I may have missed it but, Have the main filter caps been checked? My old Laney combo came to me dead, instantly blowing fuses.
The main filter caps where the cause. Laney was likely similar age.
Tested with an ESR meter and they are good.

Hawk

Replacement of CR26, dual-diode.... What effect would this have?

Is the shorted CR26 pulling too much current through the amp before a signal is applied and turning on the drivers in advance, therefore drawing too much current through the Output Transistors and therefore popping the fuse?

Basically, what is it's function and could replacing this give good odds of bringing the amp back to life?  :-\

Enzo

As I said, a shorted CR26 dual diode will only make the amp run cooler and will increase crossover distortion. It won't cause any increases in current.  In fact, shorting those bias diodes is used as a troubleshooting technique.  SHorting that diode turns OFF the drivers.  It is an OPEN CR26 that would cause excess output stage current.

This exact power amp is used in multiple amp models, it doesn't always have the output matching transformer.  The transformer does a useful job, but is not necessary to the amp function, and in times of malfunction, that transformer can obscure problems, so we disconnect it during service.

If your amp is making DC voltage, with NO load, there is nothing to draw current from the output, so it sits there with DC on the outs. When you connect a load, NOW that load draws tons of current, which loads down your power supplies.

I mentioned before not to focus on the 1.4v rails.  With the bulb limiter, some serious short in the amp lights the bulb brightly meaning the amp itself sees very low voltage.

Hawk

#72
Okay, have gone through all the resistors and diodes in power amp. Here are the readings that are suspect:


CR25 Diode Test .656/.515 . Shorted, I'd say
R92, 27K instead of 33K, as should be
R99 .824K not 1 K
R105 31.9K not 100 ohm
R107 54 Ohm not 100 Ohm
R108 54 Ohm not 100 Ohm
CR32 Diode Test .032/.032 shorted I'd say 1.
CR33 .457/.457 shorted
R118 26.5 ohms instead of 22
CR26 We know that this is already shorted.
CR23 Resistance Test 4.78M  one way, 365.5 Kohms the other (but the meter reading is still creeping up as we speak and has been doing for a while, why does that happen? Is it charging C56?) I switched to Diode Test and it reads  .709/0L so it seems good.

CR28 is also shorted .

Haven't taken out each leg of the small capacitors to measure but wonder if I should wait after replacing the bad resistor/diodes. Any thoughts? Thanks!!!!

Enzo

Your amp is blowing fuses, not just sounding distorted.  A 27k resistor in place of a 33k one will not blow fuses.

But I'll look through all this anyway.

CR25 has a resistor across it so it won't measure like a diode - but you can still see it is not shorted, shorted diodes measure close to zero.

R99 - didn't we already discuss that R97 and R99 were in parallel due to R119,R120? And didn't we calculate that exact number of ohms?

R105,R107 - they are in parallel through R113.  If R113 is not open, then around 50 ohms is what I'd expect.  I see no way to get 50 ohms at R107 f R105 is open, and 33k there is an indicator of open.  I suggest recheck R105.

R108, same as R107, parallel R106 through R114, expect 50 ohms.

CR32 has R110 and R114 across it in parallel, they make 47.33 ohms.  A diode with 47 ohms in parallel will test like shorted.

CR31,33 - VERY rare to find these bad.  Possible of course, but frankly more likely Q10,11,or 13 shorted E-C.

R118 - OK that is off a little.  If it looks like it got hot, replace it, otherwise I wouldn't sweat the difference.

CR28 - has a 22 ohm resistor across it in parallel, so it measures like a short.

CR23 -  it tests like the diode it is, trust that.  When you put it on resistance, then your meter is trying to charge up the capacitors in the circuits.  Your meter shows that as a slowly changing reading.   Get an electrolytic cap from your parts drawer and measure its resistance, and see the meter act similar.

The lesson her to learn is these things are not about parts, they are about circuits.  Every part in there is a piece of a larger system.  You cannot ignore all those parallel circuit paths.


When an amp blows fuses (or lights the bulb bright) then the first thing I look for is Emitter to Collector shorts on all the power transistors.

Hawk

Thank alot Enzo, after I fix this amp I'll be going over your notes and check out the  math and relationship between components in the circuit.

I feel at this point I've come full circle. I already pulled out the driver and output transistors and found them to be good but one--I used my Atlas DCA55 tester and it showed a short on T13.

Once I replaced the output transistors, plus the driver transistors which I had to replace due to destroying them when removing, I have only tested the amp using the bulb limiter. Is it possible to blow them when using the limiter? Can't see how as the current is so low.


I've retested the  .33 emitter transistors and my meter vacillates between .3 and .4 so hopefully that will not have an adverse effect. Should I replace them just to be sure?

I've included a pic of the output transistors. I was told by Peavey that I needed to use a matched set so I replaced 12 and 13. I used the same grey rubber insulators that came with the old transistors.

I'm going to re-test the output transistors.